Lolita, an orca at the Miami Seaquarium that had entertained guests for more than 50 years before the park bowed to public pressure and planned to release it into the ocean, died on Friday.

The orca, also known as Tokitae and Toki, had shown “serious signs of discomfort” over the past two days before she died of what was believed to be a renal issue, the Miami Seaquarium said in a post on Facebook.

“Toki was an inspiration to all who had the fortune to hear her story and especially to the Lummi nation that considered her family,” the Seaquarium said. “Those of us who have had the honor and privilege to spend time with her will forever remember her beautiful spirit.”

Under pressure from animal advocates who protested against the Seaquarium, officials there announced in February that it was preparing to return Lolita to the ocean. The Dolphin Company, which owns the aquatic park, said in March that relocating Lolita would happen in about 18 to 24 months.

Aquariums and theme parks with Killer Whales have been a source of controversy over the years, with advocates and regulators raising concerns about how marine mammals are treated in captivity. Such criticism prompted SeaWorld in 2016 to stop breeding killer whales.

In an update on Tuesday, the Seaquarium said that Lolita was “very stable and as good as she can be at 50 years of age.”

A sign of her good health, the Seaquarium said, was that she had a good appetite, eating several pounds of salmon, herring and squid. Lolita was being cared for by a team of veterinarians, who monitored her physical and mental health, the Seaquarium said on Tuesday.

It was unclear on Friday how Lolita’s condition deteriorated the week after the positive update from the Seaquarium. The Seaquarium said in a statement later that day that Lolita’s health had “deteriorated rapidly.”

“At her advanced age, her passing is not a complete surprise,” the facility said. “Yet it is a tremendous and overwhelming loss, one felt particularly by those who have loved and cared for her on a daily basis, as well as those around the world that care so much for her.”

Details about a farewell ceremony for the whale would be announced later, the Seaquarium said.

Eduardo Albor, chief executive of The Dolphin Company, said in a post on X, the social media platform formerly known as Twitter, that “not a single effort we made to give Lolita an opportunity was a waste of time & money.”

“My heart is truly broken,” Mr. Albor wrote. “Lolita captured me since 1st day. Love at first sight.”

Jim Irsay, the Indianapolis Colts owner who was involved in efforts to bring the whale to the Seattle area, said in a statement on Friday that Toki’s “story captured my heart, just as it did millions of others.”

“I was honored to be part of the team working to return her to her Indigenous home, and I take solace in knowing that we significantly improved her living conditions this past year,” Mr. Irsay said. “Her spirit and grace have touched so many.”

Still, Lolita’s death sparked renewed criticism about how she had been kept in captivity.

People for the Ethical Treatment of Animals said in a statement that Lolita had been kept in “the smallest, bleakest orca tank in the world, deprived of any semblance of a natural life” and had displayed “repetitive and abnormal behavior,” indicating “severe psychological trauma.”

Naomi Rose, a marine mammal scientist at the Animal Welfare Institute, said in a statement on Friday that “for the first time in her long life in that horrible tiny tank, she had hope,” referring to the plans to return Lolita to the ocean.

“It is a sad irony that Tokitae died now,” Dr. Rose said. “Humans failed her.”

The Seaquarium said in March that Lolita had stopped performing in 2022 as her health declined. The whale was about 20 feet long and 7,000 pounds, and was living in a 80-foot-long and 35-foot-wide tank that was no longer being exhibited.

Killer whales can live with their birth families for their entire lives. Matriarchs can live to 80 or 90 years old.